Macbook pro 'wet the bed'...what the?

I picked up my macbook pro this evening, and there was a puddle underneath it. It left a light yellow line of residue on the desk where the back vent is, the back of the vent has the same light residue, and there's a small spot of residue in the center of the back. I have a picture or two, but can't find the flash memory reader, so no "evidence" just yet.

I swear on a stack of BCS floppies that I'm not making this up, and I NEVER spilled anything onto it, around it, near it, blah blah. There are no water marks on the ceiling. Hasn't been anything liquid on my desk in weeks. I've thought it over for a good hour and I have no idea where the water came from.

This liquid had no discernable smell, although when I smelled the back of the macbook, it had a smell similar to...well...best I can think of is "a clothing iron", or maybe car coolant (which has a slightly sweet smell.) I've (gently, while off) shook it, turned it all sorts of directions, etc. Not a drop more has come out.